ASTANA, Kazakhstan — Russia, Turkey and Iran agreed Tuesday to the outlines of a plan to reinforce a cease-fire in Syria, establishing the three most significant allies of the protagonists in the conflict as guarantors to a peace process.

The deal concluded two days of talks in Kazakhstan’s capital, Astana, that drew Iran into a burgeoning alliance with Russia and Turkey over ways to secure a settlement. It set broad but vague parameters for a cease-fire enforcement mechanism and committed the three countries to jointly fight the Islamic State and Syria’s al-Qaeda affiliate. It will also provide a test of Russia’s new role as the lead power broker in efforts to secure a sustainable, long-term solution to the war.

The United States, which is not a party to the emerging peace process, said it welcomed any “actions that sustainably de-escalate violence and reduce suffering in Syria,” according to a statement issued by the State Department in Washington.

The agreement stressed that any talks on a political settlement will take place under the auspices of the United Nations in Geneva, in accordance with an existing peace process mandated by U.N. Security Council resolutions and supported by the United States.

The Syrians at the talks were also not party to the agreement and were not asked to sign on to it, underscoring the extent to which their fate is now being decided by the outside powers that have been their benefactors in the war.

Russia and Iran have been instrumental in securing President Bashar al-Assad’s survival against the rebellion seeking to unseat him, while Turkey has been the rebels’ biggest source of the supplies that have sustained their revolt.

Both the rebels and the regime indicated that they were unhappy with some of the terms of the deal, calling into question whether it can result in an enduring cease-fire. As the talks progressed, government loyalists continued to bombard the Wadi Barada area west of Damascus, Syria’s capital, drawing calls from the rebels for Russia to demonstrate its commitment to the cease-fire by pressuring the government to desist.

In a sign of how distant a real peace settlement remains, the rival Syrian delegations refused to negotiate face to face during the two-day conference at the Rixos Hotel in Astana, although they did sit in the same room for the first time during the opening session.

In any case, the Syrians played only a peripheral role at the talks. The real negotiations took place at a hotel several miles away, among the Russian, Iranian and Turkish officials who hammered out the deal.

The head of the Syrian rebel delegation, Mohammed Alloush, questioned Iran’s commitment to the cease-fire deal, citing its role in arming and funding the militias that have been behind many violations of the truce.

Bashar al-Jaafari, the head of the Syrian government delegation, described Turkey’s participation in the agreement as “negative” because of the role it has played in sending arms across its border to the rebels.

But after six years of bloodshed and 500,000 lives lost, government and opposition forces are weary and entirely dependent on their foreign backers for arms, money and, in the case of the Syrian government, men, leaving them with few options but to fall in line.

Under the agreement’s terms, Russia, Turkey and Iran are to establish an unspecified form of trilateral mechanism to enforce the cease-fire. The three powers stressed that future talks on a political settlement to the conflict are to be conducted under the guidance of the United Nations.

Perhaps most important, the deal shifted the parameters of the war by stating the parties’ determination to “fight jointly” against the Islamic State and Syria’s al-Qaeda affiliate, which are to be separated from rebel groups that participated in the talks.

That gives Iran a role in a new alliance against the Islamic State, possibly complicating any future attempts by the Trump administration to team up with Moscow against the militant organization.

It also cements Turkey’s move away from supporting the rebels’ war to topple Assad, toward a policy focused on counterterrorism.

As the final details were being worked out, the rebels’ hand was already being forced on the battlefield by the al-Qaeda affiliate Jabhat Fatah al-Sham. Fighters with the extremist group launched a major offensive in northern Idlib province against moderate rebels.

The rebels responded by declaring that they would fight back, setting the stage for a showdown between the rebels participating in the Russian-led process and the extremists who are excluded from it.